Abstract
Previous in vivo and in vitro studies have indicated that the Km for phenytoin hydroxylation is about 30 microM. Yet, the drug shows dose-dependent kinetics suggesting a Km of about 5 microM. The present studies indicate the discrepancy is not due to active transport of the drug in the hepatocyte or a decrease in the Km due to the low pO2 of the portal vein leading to uncompetitive inhibition. Studies in both hepatocytes and microsomes indicate the presence of a high affinity hydroxylase with a Km of 2 to 5 microM. These data suggest that this enzyme is the one primarily involved in the metabolism of phenytoin.
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